The use of an irradiation radiation has the disadvantage of producing significant scattered radiation through the examined object, especially in the very frequent event of a diverging radiation (conical or fan shaped). In other words, each of the detectors situated behind the object receive not only a primary radiation, arising directly from the source by a rectilinear path and having crossed through a well defined region of the object, but also a scattered radiation from an indeterminate source which affects the measurement and which it would therefore be desirable to correct.
Several methods are already used. It is in this way that the primary radiation can be measured alone if a rigorous collimation of the detectors and the source is made in order to intercept the scattered radiation, but in practice said method requires a scanning of the beam, which takes time to accomplish, and during which one has to put up with movements of the patient, if one is examining living organisms.
The opposite idea of only measuring the scattered radiation has also been proposed. To do this, one arranges a discontinuous network of absorbers, such as lead balls, between the object and the detectors, in order to locally stop the primary radiation, in such a way that the detectors located behind said absorbers only measure the scattered radiation. This process, called the “beam stop” process, therefore gives two dimensional tables or bundles of the value of scattered radiation, which one completes by interpolation between the detectors located behind the absorbers. The scattered radiation estimated in this manner is subtracted from the total radiation measured separately. This process is precise but has the disadvantage that it imposes two irradiations of the object and thus the object receives double the dose of radiation. A final example of a method for correcting the scattered radiation by material means comprises the use of anti-scattering grids, but they are only partially efficient; it is insufficient for a conical beam, where the scattered radiation may be several times greater than the primary radiation.
Finally, a certain number of digital methods exist for estimating the scattered radiation, from convolutions and deconvolutions of measurements, for example: one could also cite French patent 2 759 800 for a different, analytical digital method. Said methods are, in general, difficult to employ since they depend on parameters chosen by the user (convolutions kernel, for example) that only give good results in favorable situations, such as small areas where the scattered radiation is low, or objects with a relatively homogeneous content. No simple method exists that makes it possible, for example, to correct the scattered radiation through the thorax or other major anatomical regions, which are frequently examined but which are unfavorable for correcting the scattered radiation due to their very volume and the heterogeneity due to the presence of a complex bone structure and in which the radiation attenuation capacity is very different to that of soft tissue.
Finally, we should mention American U.S. Pat. No. 6,018,565 which describes a mixed method, using “beam stop” and convolution.